In a single series light string of 100 miniature lights, there are numerous potential causes for the failure of any individual light. When several such strings are placed on a single Christmas tree, and that hundreds of millions of sets of lights are manufactured every year, reduction of the mechanisms that can cause light failure quickly becomes a high priority.
Miniature bulbs have two conductive wires called Dumet wires that extend from inside the glass bulb, where they connect to the ends of a filament, to the outside. These Dumet wires pass through holes in a bulb holder and are bent back around the outsides of the holder. The holder is then inserted into a lamp socket carrying two electrical terminals in opposing relation on the inside of the socket wall. When the holder is properly seated in the socket, the Dumet wires are in electrical contact with those electrical terminals and pass current from one terminal through the filament to the opposing terminal. If, however, the bulb is twisted with respect to the holder, the Dumet wires may break or may be retracted far enough back into the holder to be out of electrical contact with the terminals and to thereby open the circuit.
The twisting of the bulb with respect to the holder may happen accidentally, as when a person who is accustomed to bulbs that are threaded unconsciously attempts to twist the bulb in order to withdraw it from the socket, or intentionally, as when vandals deliberately twist individual bulbs to cause them to fail. Outside lighting displays are especially subject to vandalism, as vandals will twist a number of bulbs until the whole string fails.
There has been one known attempt to solve this problem of open circuits caused when bulbs are twisted with respect to the holder, namely, the use of adhesives to glue the bulbs to their holders. However, this solution has proved unsatisfactory as the thermal expansion coefficient of the plastic, particularly when the bulb is part of an outdoor display in winter, eventually causes the bulb to loosen from its holder notwithstanding the adhesive. Accordingly, there remains a need for a way to prevent inadvertent or intentional twisting of bulbs.